What is the greatest common factor of $15$ and $45$ ? Another way to say this is: $\operatorname{gcf}(15, 45) = {?}$
The greatest common factor is the largest number that is a factor of both $15$ and $45$ The factors of $15$ are $1$ $3$ $5$ , and $15$ The factors of $45$ are $1$ $3$ $5$ $9$ $15$ , and $45$ Thus, the greatest common factor of $15$ and $45$ is $15$. $\operatorname{gcf}(15, 45) = 15$